An insulated gate bipolar transistor (IGBT) or metal oxide semiconductor field effect transistor (MOSFET) needs to be protected against shorts and overcurrent conditions. Too large of a transistor current, defined as the current between collector and emitter in the case of IGBTs and the drain to source current in MOSFETs, can damage the transistor. Monitoring circuits have been used to measure a device voltage between the collector and emitter or source and drain in prior attempts to determine whether a short or overcurrent condition exists during an on state of the transistors. The monitoring circuits compare the device voltage to a reference voltage to enable the transistor to be turned off should the device voltage exceed the reference voltage. Such monitoring circuits are a last resort of protection when other lower level of protection circuits fail to protect the transistor from high current.
Immediately after turn on of these types of transistors, there may be considerable ringing on the collector or source due to parasitic capacitances and parasitic wiring inductance. The monitoring circuit is blanked during this period to allow the ringing to clear and prevent false tripping of IGBT activated by the monitoring circuit. The time duration between the instant the transistor is turned on and the activation of the monitoring circuit is referred to as a blanking time. The blanking time may vary from 1 to 4 μsec in various transistors. The transistor is not protected from shorts or overcurrent during this time, and may be damaged from high currents. Where such transistors are used in switching applications, where the transistors are turned on and off quickly, the blanking time can be a significant amount of time that the transistors are unprotected. There is a need to protect such transistors during the blanking time.